/*
 * ProGuard -- shrinking, optimization, obfuscation, and preverification of Java bytecode.
 *
 * Copyright (c) 2002-2018 GuardSquare NV
 *
 * This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public
 * License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later
 * version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
 * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along with this program; if not, write to the Free
 * Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 */
package proguard.optimize.peephole;

import proguard.classfile.*;
import proguard.classfile.attribute.CodeAttribute;
import proguard.classfile.editor.CodeAttributeEditor;
import proguard.classfile.instruction.*;
import proguard.classfile.instruction.visitor.InstructionVisitor;
import proguard.classfile.util.SimplifiedVisitor;

/**
 * This InstructionVisitor simplifies unconditional branches to other unconditional branches.
 *
 * @author Eric Lafortune
 */
public class GotoGotoReplacer extends SimplifiedVisitor implements InstructionVisitor {
    private final CodeAttributeEditor codeAttributeEditor;
    private final InstructionVisitor extraInstructionVisitor;

    /**
     * Creates a new GotoGotoReplacer.
     * 
     * @param codeAttributeEditor a code editor that can be used for accumulating changes to the code.
     */
    public GotoGotoReplacer(CodeAttributeEditor codeAttributeEditor) {
        this(codeAttributeEditor, null);
    }

    /**
     * Creates a new GotoGotoReplacer.
     * 
     * @param codeAttributeEditor a code editor that can be used for accumulating changes to the code.
     * @param extraInstructionVisitor an optional extra visitor for all replaced goto instructions.
     */
    public GotoGotoReplacer(CodeAttributeEditor codeAttributeEditor, InstructionVisitor extraInstructionVisitor) {
        this.codeAttributeEditor = codeAttributeEditor;
        this.extraInstructionVisitor = extraInstructionVisitor;
    }

    // Implementations for InstructionVisitor.

    public void visitAnyInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset,
        Instruction instruction) {}

    public void visitBranchInstruction(Clazz clazz, Method method, CodeAttribute codeAttribute, int offset,
        BranchInstruction branchInstruction) {
        // Check if the instruction is an unconditional goto instruction.
        byte opcode = branchInstruction.opcode;
        if (opcode == InstructionConstants.OP_GOTO || opcode == InstructionConstants.OP_GOTO_W) {
            // Check if the goto instruction points to another simple goto
            // instruction.
            int branchOffset = branchInstruction.branchOffset;
            int targetOffset = offset + branchOffset;

            if (branchOffset != 0 && branchOffset != branchInstruction.length(offset)
                && !codeAttributeEditor.isModified(offset) && !codeAttributeEditor.isModified(targetOffset)) {
                Instruction targetInstruction = InstructionFactory.create(codeAttribute.code, targetOffset);

                if (targetInstruction.opcode == InstructionConstants.OP_GOTO) {
                    // Simplify the goto instruction.
                    int targetBranchOffset = ((BranchInstruction)targetInstruction).branchOffset;

                    Instruction newBranchInstruction =
                        new BranchInstruction(opcode, (branchOffset + targetBranchOffset));
                    codeAttributeEditor.replaceInstruction(offset, newBranchInstruction);

                    // Visit the instruction, if required.
                    if (extraInstructionVisitor != null) {
                        extraInstructionVisitor.visitBranchInstruction(clazz, method, codeAttribute, offset,
                            branchInstruction);
                    }
                }
            }
        }
    }
}
